Bumper reinforcing materials or indoor impact absorbing materials are directly associated with the safety of passengers in vehicle collisions, and therefore the ultra high strength hot rolled steel plates having a tensile strength of 780 MPa or more have been widely used as the reinforcing/absorbing materials. Also, the reinforcing/absorbing materials should have high elongation as well as high tensile strength, and its excellent hole expansibility is required to improve formability of a flange unit or a part coupling unit.
For the purpose of coping with regulation for increasingly serious environmental pollution problems, high strength steel has been increasingly used in high strength parts to improve fuel efficiency, and therefore there has been an increasing attempt to commercialize a high strength steel having a tensile strength of 780 MPa or more.
Representative examples of the high strength steel used for automobiles include a multi-phase steel, dual-phase (DP) steel, a transformation induced plasticity (TRIP) steel and a twin induced plasticity (TWIP) steel.
In general, a method for manufacturing a plate sheet is divided into a re-heating process for re-employing segregated components of manufactured slabs, a hot rolling process for rolling the slabs into plates of a final thickness, and a cooling process for cooling/winding the hot-rolled plate at room temperature. Here, the slabs taken out from a heating furnace are rolled in an austenite zone, and austenite is then transformed into martensite at a lower finish cooling temperature than a martensite start (Ms) temperature in the cooling process. At this time, the resultant steel is referred to as a dual-phase steel.
The dual-phase steel has an increasing strength with the increase in the ratio of martensite over the entire structure, and also has an increasing ductility with the increases in the ratio of ferrite. In this case, when the ratio of martensite is increased to enhance its strength, the ratio of ferrite is relatively decreased, which leads to the deteriorated ductility. And, the dual-phase steel has a problem that its cooling rate should be increased to form martensite at low temperature.
As described in the method, the austenite is formed in the rolling process, and the ferrite, the martensite, some of the bainite and a mixed martensite/austenite phase are formed at room temperature by controlling the cooling rate, the finish cooling temperature and so on in the cooling process. The resultant steel that improves strength and ductility of the transformation induced plasticity steel is a multi-phase steel.
The multi-phase steel does not have a yield ratio characteristic caused by the martensite transformation, and therefore the multi-phase steel has been widely used in a variety of application fields since it has excellent weldability due to the use of a relatively low amount of added alloy elements, and also has high yield strength although its formability is rather unfavorable because of the high yield strength.
Also, after the austenite, the austenite or the ferrite dual phase is formed in the rolling process, and then heat-treated in the bainite transformation temperature range by controlling the cooling rate and the finish cooling temperature in the cooling process, the transformation induced plasticity steel may be manufactured when the condensed austenite remains metastable at room temperature in addition to the bainite transformation. Amongst the currently commercially available steels, the transformation induced plasticity steel has the most excellent strength and elongation balance (strength*elongation).
Considering the steels that are under the commercial use stage, the twin induced plasticity steel has the most excellent strength*elongation balance. The twin induced plasticity steel is a steel whose strain hardening property is improved, thereby suppress necking and improve elongation, by adjusting components such as manganese, carbon and aluminum to obtain a stable austenite single phase and using dislocation and twin systems as the transformation apparatus during the phase transformation.
However, when the martensite is subject to the strain hardening process, boundaries of soft matrix phases and hard martensite phases are sufficient to form vacancies during the phase transformation or processing process, and therefore its strength vs. elongation is excellent but its hole expansibility is poor.
The transformation induced plasticity steel has a low burring workability since vacancies are also formed in boundaries of transformation induced martensite and soft matrix phase during the phase transformation. The twin induced plasticity steel has the same or similar level of hole expansibility, compared to the ultra high strength steel (dual-phase steel, transformation induced plasticity steel, etc.) of the same strength, which is considered to be associated with the high strain hardening rate caused by the twin.